The present invention relates to a hydraulic brake system for automotive vehicles comprising a traction slip control system including a master cylinder and a pedal-operated servo system for actuating the master brake cylinder. The booster chamber of the servo system is placed in communication with a pressure source at any pedal position. An electronic analyzer generates a signal when one of the wheels tends to spin.
A brake system of the afore-mentioned type has been described, for example, in German patent document DE-OS 36 35 0 054. This type of brake system for traction slip control evolved from a brake system for brake slip control. This is because means, such as sensors, evaluation units, valves, etc., which are suitable to prevent the wheels from locking during deceleration, also can be employed to prevent the wheels from spinning in the start-up phase--i.e., traction slip control
However, in brake systems of this type a problem is presented. That is, to avoid locking of a wheel, the pressure in the wheel brake cylinders must be rapidly decreased which is accomplished by opening the corresponding outlet valve so that pressure fluid escapes into a reservoir. However, to prevent wheel spinning, first a pressure must be built up. For a conventional system, this means that the pumps are actuated to deliver pressure fluid to the brake conduits. However, it takes a significant period of time before the pump reaches its full discharge capacity which means that the pressure in the wheel brake cylinders cannot be sufficiently rapidly developed as is required to safely prevent the wheel from racing. The noted German patent document, therefore, suggests associating accumulators with the pressure conduits of the pumps, thereby, in the initial phase of the pump, supplying pressure fluid to the in-flow conduits. Disadvantageously, monitoring of such accumulators requires substantial mechanical efforts. For example, both a release valve and a pressure switch for monitoring the loading operation of the accumulator must be provided. Moreover, additional valves are required to enable the accumulator to be charged. In addition, valves must be inserted into the brake conduits to prevent the pressure fluid from passing from the accumulator, through the non-actuated master brake cylinder, into the reservoir.
It is therefore an object of the present invention to provide the noted brake system such that the initial phase of the pump, in a traction slip control, and despite simplified mechanical measures, adequately can be provided.